Phylum: Mollusks
Three Classes of Mollusks
1. Class Gastropoda – snails, slugs2. Class Bivalvia – clams, oysters, mussels,
scallops3. Class Cephalopoda – octopi, squids and
cuttlefishes
Basic information
1. Over 100,000 living species 2. Marine, fresh water, and land3. Most Bilateral symmetry
Body Plan
• Divided into two regions: 1.head/foot 2.visceral mass
Head/foot
• Head (contains mouth and variety of sensory structures)
• Foot (muscular organ used for locomotion)
Visceral Mass1. heart2. digestion3. excretion4. reproduction
Mantle
1. Covers and protects the visceral mass2. Secretes shell
mantle cavity
• Location of gills• Space between the mantle and the visceral
mass
nervous system
• Ganglia: paired cluster of nerve cells1. locomotion 2.feeding 3.process sensory information (light, touch)
Feeding
• Radula: flexible, tongue like strip of tissue covered with tough abrasive teeth that point backward
Feature Gastropoda Bivalvia Cephalopoda
External Shell 0-1 Two None (except nautilus)
Head Yes No Yes
Radula Yes No Yes
Locomotion Crawl sessile Rapid swimming
Class Gastropoda
Basic Information
• Most diverse class of mollusks• 90,000 species• Snails, abalones, conches= single shell• Slugs and nudibranchs= no shell
Torsion
• Visceral mass twists around 180° in relation to the head
• Twisting results in mantle cavity, gills, and anus to the front of the animal
• Gastropod is now able to with drawl its head into mantle cavity when threatened
Movement
• foot secrets a substance allowing animal to glide over surfaces (Slime Trail)
Open circulatory system
• HEMOLYMPH (blood in an organism with open circulation) does not remain in vessels
1.Collected from gills or lungs 2.Pumped through heart3.Released directly into spaces in the tissues
a) Fluid filled spaces or blood cavity
Bivalvia: Clams
Other Bivalvia: scallops, mussels, oysters
Characteristics
1. Sessile2. Filter feeders3. No radula4. No cephalization5. Aquatic
Anatomy of Valves
• Shell is divided into two halves (valves)
• Connected by a hinge• Adductor muscles
– Contract: close valves– Relax: open vlaves
Clam Sensory information
• 3 pairs of ganglia – mouth (cerebral)– digestive system (visceral)– Foot (pedal)
Foot
• Foot helps burrow in the sand or mud
Water flow• water enters through
incurrent siphon• Water exits through
excurrent siphon
Steps in digestion: Filter Feeders
1. Cilia in gills set up water current2. Gills filter water for small organisms3. Palps: flaplike structures that surround &
guide food into the clam's mouth4. Food then enters the mouth
Steps in Digestion: Filter Feeders
4. Stomach: Digestion begins5. Digestive glands: digested particles are
absorbed6. Intestines: collects and removes digestive
wastes7. Waste are passed through the rectum and
excreted through anus
Function of Gills
1. Trap food particles2. Exchange oxygen and carbon dioxide
Growth rate
• Umbo oldest part of the clam
• Growth rings
CLASS CEPHALOPODAMost advanced
class
Cephalopod “Head-foot”
• foot is concentrated in the head region• foot is modified into arms and tentacles
equipped with suckers• Foot also forms funnel (siphon) for expelling
water, allowing movement by "jet propulsion"
Squid
Cuttlefish
The major distinction between the squid and octopus
Squid suction cups are armed with hooks or sucker rings (or a combination ofthe two). Octopus have simple suction cups without secondary armature.
Tentacle club of Architeuthis, showing circular-saw-like sucker rings.
tentacle club of Mesonychoteuthis, with swiveling hooks.
Squid Suction Cups
Profile of Mesonychoteuthis tentacle club, showing hooks.
Suction cups of Haliphron atlanticus, the giant gelatinous octopus (the world's largest species of octopus), lacking secondary armature.
Giant SquidCan reach length of up to 60 feet and weight of more than 3.5 tons.
Architeuthis dux
Nautilus
Locomotion
• Jet propulsion by using siphon to force water out
• Crawling – octopus*Also uses siphon
External Features
• Octopus: 8 arms with either one or two rows of suction cups (but never hooks or sucker rings),
• Squid/cuttlefish: 10 appendages containing suction cups (contain either hooks or sucker rings)
• 8 arms• 2 tentacles
Ink–All except nautilus have ink sack which empties into rectum; ink contains the pigment melanin (same pigment as human skin)
–Released when the animal is alarmed. The animal quickly departs from the scene leaving the ink as a decoy to the predator.
Color Changes• Chromatophores – pigment cells that
expand and contract to produce color change. – Used as danger signals, protective coloring,
and for courtship.
The Blue Ringed Octopus (found in shallow coral and rock pools of Australia): It’s poisonous saliva is 10,000 more potent than cyanide. This octopus is only the size of a golf ball but carries enough poison to kill 26 humans in minutes.
Feeding
1. Fish, other mollusks, crustaceans, worms
2. Beak like jaws and radula tear prey into pieces
3. Octopus and Cuttlefish have poison in saliva
Circulatory
• Closed circulatory system• Blood delivers oxygen and
nutrients directly to organs through veins and arteries
Nervous System
• Well-developed brain; eyes which are similar in construction to vertebrate eyes
• FYI: The giant squid has the largest eye of any animal, either living or extinct. In a 55-foot specimen the diameter was 15.74 inches. In comparison, a blue whale's eye has the diameter of 4.70 inches, and humans have an eye diameter of .94 inches.
Reproduction
1. Male or Female2. The male uses arm to take sperm from
own mantle cavity and insert into females mantle cavity
3. The female lays ~100 eggs and guards them until they hatch (approx 50 days)
4. Frequently the mother dies soon after the eggs hatch because during the guarding of the eggs she is unable to eat.
Squid Vs Octopus